Due to threats of military or terrorist attacks, public and private shelters are provided to protect the population from dangers mechanical in nature, e.g. falling walls, projectiles, blast and flying debris. Many homes are also provided with security rooms for the same purpose. In recent decades it is also considered prudent to provide protection also against nuclear, chemical and biological (NBC) agents, by provision of NBC air filtration systems including blowers, which create an above-atmospheric pressure inside the protected space and provide cleaned, purified and filtered air. The above-atmospheric pressure is required in order to ensure that no unfiltered air can enter the protected space through ever-present leaks in the walls of the shelter, the door and the window seals.
As during war and emergency situations the electric power supply may be interrupted for hours or even days, a mains-driven electric blower can not be relied upon to push or pull air through a filter and into the protected space. A possible solution is to use power generators or batteries to provide electricity. However as power generators and batteries are kept for many years often without the required test and maintenance procedures, they may not be in operative condition when required. From this consideration the most reliable power source for the blower is a geared hand crank. The hand crank is however feasible only if no great effort is involved in its operation, wherefore a filter having a low flow resistance is essential.
The required capacity of the air purification system is a function of the number of persons expected to be present in the protected space. The quantity of air needed by each person is in the range of 2 to 10 m3/h. Thus a family of 6 should be provided with up to 60 cubic meters of fresh air per hour.
While activated carbon filters have long been known for purification of liquids and of gases, little is known about active carbon gas/gas filters optimized for low flow resistance. The state of the art can be assessed by review of recent U.S. Patents.
In U.S. Pat. No. 5,198,004 Vollhardt discloses an activated carbon filter for removing toxic substances such as dioxins and furans from flue gases. The carbon is mixed with 30%–40% water and the gases are bubbled through the mixture before entry into a smokestack.
Tang et al. describe a self-supporting porous filter for automobile use in U.S. Pat. No. 5,332,426. Adsorbent particles and particulate thermoplastic binder particles are fused into a desired shape.
A process for manufacturing a carbon particle filter is disclosed by Bandel et al. in U.S. Pat. No. 5,391,338. A layered or woven porous ceramic material is used in combination with a fabric which is burnt away after formation of the filter body.
A filter for the selective separation of organic material from an organic phase is disclosed by Nikolskaja in U.S. Pat. No. 5,656,069. A filter material is prepared in a process wherein polyvinyl alcohol powder is mixed with active carbon powder and water in a process including compression in a mold and multi-stage heating.
Chinn et al. in U.S. Pat. No. 6,332,916 disclose a method for manufacturing an activated carbon filter containing a transition metal such as copper, cobalt, chromium or silver. A pore network is formed by a process wherein the first step is exchanging the metal with a cellulose ion exchange material.
The NBC filter which is presently in widest use in the USA is the M98 Gas Filter which has a cylindrical shape and is specified in the following table. This filter has the disadvantage of a pressure drop exceeding 1000 Pa at the stated air velocity.
M98 Gas Filter[inch][mm]Length10.20259.1Inside diameter16.69423.9Outside diameter21.43544.3Average diameter19.06484.1Filter bed thickness2.3760.2[sq. inch][cm2]Nominal face size610.763,940.4[cfm][m3/h]Airflow nominal200339.8[inch/s][cm/s]Air Velocity9.4324.0[IWG][Pa]Airflow resistance4.51119(when filter is new)Gas Adsorption:[mg × min/m3]DMMP CT-value>300,000(when filter is new)CK CT-value (new)>110,000Bulk density0.623